1. Field of the Invention
The present invention generally relates to a pulse width modulation device and an image forming device.
2. Description of the Related Art
FIG. 26 is a drawing illustrating an exemplary configuration of a conventional image forming apparatus such as a laser printer or a digital copier. As shown in FIG. 26, a laser beam emitted from a semiconductor laser unit 1009 is deflected by a rotating polygon mirror 1003, passes through a scanning lens 1002, and forms a light spot on a photoconductor 1001 that is a target object to be scanned, thereby exposing the photoconductor 1001 and forming an electrostatic latent image. A photodetector 1004 detects the scanning beam for each line.
A phase-locked loop 1006 receives a clock signal from a clock generating circuit 1005, generates a phase-synchronized image clock signal (pixel clock signal) for each line based on an output signal from the photodetector 1004, and supplies the generated image clock signal to an image processing unit 1007 and a laser driving circuit 1008. The laser driving circuit 1008 controls the light emitting time of the semiconductor laser unit 1009 according to image data generated by the image processing unit 1007 and the phase-synchronized image clock signal generated by the phase-locked loop 1006 for each line and thereby controls the formation of an electrostatic latent image on the photoconductor 1001.
In a scanning optical system as described above, variation in scanning speed leads to irregularity in an image and therefore degrades image quality. Especially, when forming a color image, variation in scanning speed causes misalignment of color dots in the main scanning direction and thereby causes color shift and reduces color reproducibility and image resolution. Therefore, to improve image quality, it is necessary to reduce the variation in scanning speed.
Major types of scanning speed variation (error in scanning speed) are described below.
(1) Error in Scanning Speed Relating to Reflecting Surfaces of Polygon Mirror
Error in scanning speed may be caused by difference in distance of the reflecting surfaces of a deflector such as a polygon mirror from the rotation shaft (decentering of the axis of a polygon mirror) and difference in precision of the reflecting surfaces. Error in scanning speed relating to the above causes occurs periodically every several scan lines (for example, the number of scan lines corresponding to the number of reflecting surfaces of a polygon mirror).
(2) Error in Average Scanning Speed
Average scanning speed is an average of scanning speeds of the reflecting surfaces of a polygon mirror. Error in average scanning speed is caused by, for example, variation in rotational speed of a polygon mirror and various changes in a scanning optical system caused by environmental changes in temperature, humidity, vibration, and so on. Also, error in average scanning speed may be caused by chromatic aberration in a scanning optical system that occurs when the oscillation wavelength of a semiconductor laser, or a light source, changes because of, for example, temperature change. Error in average scanning speed is moderate compared to other types.
(3) Error in Scanning Speed Relating to Light Source
This type of scanning speed error occurs in a multi-beam optical system including multiple light sources, for example, a semiconductor laser array, where multiple light beams are scanned by the same scanning optical system. Such scanning speed error occurs because of chromatic aberration in a scanning optical system that is caused by different oscillation wavelengths of the light sources. Also, this type of scanning speed error may be caused by inaccurate assembly of multiple light sources. Meanwhile, the scanning speed error as described in (2) may also vary depending on the light source, since the degree of change in oscillation wavelength differs depending on the light source.
(4) Error in Scanning Speed Relating to Scanning Optical System
In an image forming apparatus including multiple photoconductors and scanning optical systems and configured to form a color image, the difference in scanning speeds of the scanning optical systems greatly affects the image quality. The difference in scanning speeds of scanning optical systems may be caused by inaccurate production and assembly of parts in the scanning optical systems and deformation of the parts over time. Also, because of different characteristics of the light sources in the scanning optical systems, scanning speed error as described in (3) may also occur. In this case, scanning speed error as described in (1) and (2) occurs in each of the scanning optical systems and the average scanning speed of the scanning optical systems also fluctuates. There is an image forming apparatus in which some units such as a polygon mirror are shared by multiple scanning optical systems as common units. Even in this case, since the paths of light beams from the light sources to the photoconductors are different, scanning speed error as described in (4) may also occur.
Patent document 1 discloses a method of correcting the error in scanning speed by changing the frequency of a pixel clock signal depending on the scanning speed. In the disclosed method, the frequency of an oscillator for generating a pixel clock signal is controlled (phase-locked-loop (PLL) controlled) so that the count of cycles of the pixel clock signal between the start and end of scanning becomes a specified value.
However, the disclosed method has a disadvantage as described below. In the disclosed method, the frequency of the reference clock signal used for phase comparison corresponds to one scan line and is therefore far lower than (one in several thousands to one in tens of thousands) that of the pixel clock to be generated. Therefore, it is difficult to achieve enough open loop gain of the PLL and to accurately control the frequency of the pixel clock signal. Also, since the frequency of the pixel clock signal is easily affected by disturbance, it is difficult to accurately generate a pixel clock signal. Further, to reduce the difference in scanning speed of the reflecting surfaces of a polygon mirror using the disclosed method, it is necessary to change the control voltage for a voltage-controlled oscillator (VCO) for each scan. With such a method, it takes a long time for the clock frequency to become stable and therefore it takes a long time to generate a pixel clock signal.
Patent document 2 discloses a method of correcting the error in scanning speed by controlling the phase of a pixel clock signal based on a generated high frequency clock signal. In the method disclosed in patent document 2, the phase of a pixel clock signal is controlled so that the count of cycles of the high frequency clock signal between the start and end of scanning becomes a specified value. The high frequency clock signal is accurately generated based on an accurate reference clock signal from, for example, a crystal oscillator. Using such an accurate high frequency clock signal for the phase control of a pixel clock signal makes it possible to accurately generate the pixel clock signal.
However, to correct the error in scanning speed by controlling the phase of a pixel clock signal, it is necessary to generate phase control data for one scan line. Also, to reduce local deviation caused by the phase change of the pixel clock signal and thereby to accurately generate the pixel clock, it is necessary to perform high-resolution phase control. Accordingly, the size of the phase control data becomes large and it is difficult to accurately generate such a large amount of phase control data at high speed. Also, to reduce the difference in scanning speeds of the reflecting surfaces of a polygon mirror using the disclosed method, it is necessary to generate the phase control data for each reflecting surface. Therefore, in this case, the amount of phase control data increases further and it is very difficult to accurately generate such a huge amount of phase control data at high speed. Further, inaccurate production and assembly of parts in a scanning optical system may cause the scanning speed to fluctuate even during the scanning of a line.
(5) Nonlinear Error in Scanning Speed
FIG. 27A is a graph showing exemplary nonlinear error in scanning speed during the scanning of a line. In FIG. 27A, the horizontal axis x shows positions X in a scan line and the vertical axis shows scanning speeds V (X) at the positions X. Also, a one-dot broken line Vavg shows the average scanning speed during the scanning of a line. When the scanning speed fluctuates as shown in FIG. 27A, deviations Δ from a desired value, which is a value when the scanning speed is constant, become as shown by the solid line in FIG. 27B. The deviations Δ indicate misalignment of dots and cause degradation of image quality. In FIG. 27B, the dotted line shows the deviations Δ when the line is scanned in a direction from the position X2 to the position X1. As shown in FIGS. 27A and 27B, when scanning is performed in both directions in a scanning optical system that causes asymmetrical misalignment of dots with respect to the center of scanning, color shift increases and image quality is greatly degraded. Also, the degree and distribution of the deviations Δ may vary depending on the preciseness of each reflecting surface of a polygon mirror. Further, the degree and distribution of the deviations Δ may vary depending on a scanning optical system.
Patent document 3 discloses a method of reducing the nonlinear error in scanning speed by modulating the frequency of a pixel clock signal according to the position in a scan line. However, since the method disclosed in patent document 3 uses a conventional method for generating the center frequency of a pixel clock signal, as described above, it is difficult to generate an accurate pixel clock signal and to effectively correct the error in scanning speed. Thus, the method disclosed in patent document 3 is not sufficient to improve image quality.
[Patent document 1] Japanese Patent Application Publication No. 2001-183600
[Patent document 2] Japanese Patent Application Publication No. 2004-262101
[Patent document 3] Japanese Patent Application Publication No. 2000-152001